One of the most well known global navigation satellite systems (GNSSs) is the Global Positioning System (GPS). Other GNSSs include, for example, the GLObal NAvigation Satellite System (GLONASS) and the European Galileo positioning system (Galileo). A GNSS may be used in conjunction with a receiver device (e.g., a standalone GNSS navigation device or a GNSS-enabled mobile receiver device) to achieve a plurality of functions. For example, GPS mobile receivers may assist users in determining their exact location, navigating roads and locating a destination, and viewing a map of a surrounding area. While traditional stand-alone navigation devices still exist, technology now allows navigation devices to be a part of other mobile receivers such as cellular phones or personal data assistants (PDAs).
Navigation devices and other mobile receivers may receive assistance data from an assistance server in addition to receiving signals from the various satellites of the various GNSSs. Assistance data may include, for example, data instructing the mobile receiver where to look for GNSS signals (e.g., satellite positions in the sky, Doppler frequency of the satellite signal, etc). Additionally, assistance data provided to the receiver may include a recent ephemeris for a GNSS satellite (e.g., a recent set of orbit, clock, and/or almanac parameters for the satellite). Assistance data may also include corrections made to an ephemeris, or a confirmation of ephemeris data, to allow the receiver to more quickly and accurately determine its position. Thus, assistance data may allow receivers to gain certain advantages, such as detecting GNSS signals quicker than without the assistance data and detecting weaker signals.
GNSS-based positioning requires that the GNSS receiver has knowledge of the satellite navigation models used for the data. A navigation model refers to a format for the parameterization of a type of satellite data, such as orbit model. That is, satellites in one GNSS may transmit orbital parameters according to a first format, while satellites in another GNSS may transmit orbital parameters in a second format. Additional types of satellite data (e.g., clock parameters, almanac parameters) may have additional different sets of possible formats in different GNSSs. In the case of a stand-alone GNSS, a mobile receiver obtains the navigation models as well as almanacs from the satellite broadcasts. However, in an assisted GNSS scheme, the receiver may obtain a copy of the navigation models and almanac from one or more assistance servers via a separate ground-based communication network (e.g., cellular network, IP network).
Assisted GNSS allows for the possibility that GNSS receivers may receive satellite data from assistance servers in non-native formats. To illustrate, a stand-alone receiver in a GLONASS GNSS receives its GNSS data from the satellite in the GLONASS native format (i.e., GLONASS parameterization). However, if the same GLONASS satellite data is provided to the receiver via an assistance server having additional navigation models, then the assistance server may be able to provide the receiver with the data in an alternative format. This may be potentially beneficial, for example, if a non-native navigation model has a longer data validity period than the native format. As an example, Wide-Area Reference Networks follow GNSS satellites and their broadcast signals. Such networks allow for modeling the satellite orbits very accurately and predicting orbits as much as several weeks in advance. Therefore, it is possible to take such a trajectory prediction for an SV in the first navigation system (e.g., GLONASS) and fit the parameters into a navigation model natively used in a second navigation system (e.g., GPS NAV Keplerian parameterization) so that the trajectory described by the parameterization complies with the predicted orbit for the said SV in the first navigation system. In this example, instead of having to obtain an updated navigation model for the GLONASS SV every 30 minutes (the nominal validity period of the GLONASS model), when using native model, the update rate can be dropped to 4 hours (the nominal validity period of the GPS NAV Keplerian model).
However, this scheme of requesting assistance data from assistance servers in a non-native format may be problematic. For example, when the receiver requests assistance data, it does not know which navigation models are supported by the assistance server. Similarly, assistance servers do not know what navigation models can be utilized by the receiver. If an assistance server does not support a requested navigation type, it might simply return an error to the receiver, or it may default by providing the data to the receiver in the native format. These scenarios may be problematic and inefficient, especially in modern systems having more and more GNSSs using more and more navigation models. Furthermore, International GNSS Services (IGSs) and other services may provide navigation models to assistance servers in additional formats that are not native to any GNSS.
Hence, there is a need for additional devices and methods for requesting and providing assistance data from an assistance server to a receiver in a GNSS.